In recent years, video cameras and electronic cameras have become very popular. These cameras use CCD type or CMOS type solid-state imaging devices.
Among these, a CMOS type image sensor has an imaging pixel portion configured by a plurality of pixels arranged in an m×n matrix and a peripheral circuit portion arranged at the periphery of this imaging pixel portion on one semiconductor chip. The CMOS type image sensor has, in each pixel of the imaging pixel portion, a photo-electric conversion element (photo-diode), a floating diffusion portion (FD portion), and a reading circuit comprised of various types of MOS transistors for transfer, amplification, etc., photo-electrically converts light incident upon each pixel by the photo-diode to generate a signal charge, transfers the signal charge to the FD portion by a transfer transistor, detects potential fluctuation of this FD portion by an amplifier transistor, converts this to an electric signal, amplifies the same, and thereby outputs a signal for each pixel from a signal line to the peripheral circuit portion.
Further, the peripheral circuit portion is provided with a signal processing circuit for applying a predetermined signal processing, for example, correlated double sampling (CDS), gain control, or A/D conversion to the pixel signals from the imaging pixel portion and a drive control circuit for driving each pixel of the imaging pixel portion and controlling the output of the pixel signal, for example, a vertical or horizontal scanner or a timing generator (TG).
In the CMOS type image sensor as explained above, a thinning out read operation making good use of the random access property providing a reading circuit for the photo-diode for each pixel is possible.
This thinning out read operation makes it possible to eliminate waste and lower power consumption in for example a digital camera equipped with a liquid crystal monitor by outputting only the information of the thinned out pixels in accordance with the resolution of the liquid crystal monitor when monitoring occupying most of the time even if the information of all pixels is output when acquiring the captured image immediately after shutter operation.
FIG. 1 is a view for explaining a ⅓ thinning out operation when outputting one row of signals per three rows as an example of the thinning out reading mode.
FIG. 1 diagrammatically shows a pixel array arranging pixels in a matrix in a sensor portion (imaging pixel portion) of a CMOS type image sensor. In FIG. 1, the outer frame indicates a sensor portion (imaging pixel portion) 1. Each grid box in this indicates a pixel 2. Only pixels indicated by hatching among the pixels are pixels read out in the thinning out reading mode. The other pixels are pixels which are not read out in the thinning out reading mode.
A pixel signal is read in units of pixel rows by the operation of for example a vertical shift register. In the usual all pixel reading operation mode, the read operation is carried out by sequentially selecting all pixel rows of the sensor portion (imaging pixel portion).
In the thinning out reading mode, the operation is carried out by sequentially selecting only the hatched pixel rows in the imaging pixel portion to perform the reading. The remaining two pixel rows of the three rows are not selected.
When scanning by such method, however, it was seen that the image characteristic deteriorated after use over a long time. The reason for that was there is a difference in the degrees of deterioration of the transistors of pixels or the driver transistors for driving the pixel interconnects between the rows to be thinned out and the rows not to be thinned out.
At the time of all pixel output, the drive frequencies of the transistors of all rows are equal, so no difference occurs in the degrees of deterioration of the same.
At the time of a thinning out operation, however, the drive frequency of a transistor is different between a row which is thinned out and a row which is not thinned out, therefore when the thinning out operation is carried out for a long time, the degrees of deterioration of the transistors of the different rows become different.
A pixel is an analog circuit handling signals with a precision of 0.1 mV, therefore when the characteristic of the transistor changes even a little, it appears as a change in characteristic of the pixel. Especially, when the degree of deterioration of the transistor characteristic is different for each row in the thinning out operation, it becomes the deterioration of the image characteristic seen by the naked eye as periodical lateral stripes at all pixel output.